In many power circuit applications having series connected loads it is desirable to provide a means wherein failure of one of the loads resulting in an open circuit will not interrupt the power to the remaining loads.
An example of an application is an airport lighting system wherein the loads are lamps located atop towers.
A circuit wherein lamps are connected in series and remotely located from a power source requires only two wires to connect the lamps to the power source. However, failure of a lamp resulting in an open circuit will interrupt the operation of the circuit. To avoid this problem, many circuits have incorporated various forms of shorting circuits which shunt each lamp. When a lamp fails resulting in an open circuit, the shorting circuit is activated and places a short across the failed lamp thereby completing the circuit and allowing current to flow to the remaining lamps. Booth et al U.S. Pat. No. 1,024,495 and Stier U.S. Pat. No. 2,809,329 disclose series connected lamps shunted by normally open shorting circuits. However, use of present mechanically held shorting devices in airport lighting systems is expensive and the failure rate of such shorting devices is relatively high.
Isolation transformers are typically used to distribute power from a main power source to the lamps. To avoid the cost and high failure rate of present shorting circuits, each lamp may be connected to a different isolation transformer secondary winding. The transformer primary windings are connected in series to the main power source. In this circuit, each lamp is connected to a transformer secondary winding by two conductors. In the event a lamp fails resulting in an open circuit, the power to the other lamps is not interrupted.
However, one disadvantage of this circuit can be seen where the lamps are located atop approach towers. Such towers must be frangible to enable the tower to collapse under impact from a plane in flight to minimize damage to the plane and injury to occupants therein. In this circuit, 2N wires must be run up the tower, where N is the number of lamps. Such a large number of wires results in a less frangible tower.
Another disadvantage is that a larger number of wires increases the cost. This is apparent when the height of the tower is taken into consideration. If five lamps are located atop the tower, for example, the circuit would require ten wires extending to the top of the tower.
Another power circuit arrangement is shown in Jacob U.S. Pat. No. 3,969,649. Jacob discloses a bicycle lighting system including two lamps connected in series across a winding of a dynamo. An impedance is connected between an internal tap of the winding and a junction point between the lamps. The impedance is selected to establish system equilibrium whereby the lamp junction point and tap are maintained at the same potential under normal operating conditions despite variations in dynamo and lamp resistance with bicycle speed. If a lamp fails resulting in an open circuit, the power to the remaining lamp is not interrupted.
This Jacob circuit eliminates the need for shorting devices shunting each lamp. However, the dynamo winding and impedance must be selected for a given set of lamps having particular electrical ratings. If one or both lamps are exchanged for a lamp having a different electrical rating, the system equilibrium will be offset. Thus, the impedance and/or dynamo must be replaced by a different impedance and dynamo to reestablish system equilibrium.